1. Technical Field
The present invention relates to a braking system for vehicles, particularly commercial vehicles.
2. State of the Art
German patent document DE 2 726 640 A1 shows a braking system for vehicles having a service brake valve, which is connected by way of pneumatic lines with ABS-valves of a vehicle axle. The ABS-valves are each connected with an assigned pneumatic/hydraulic converter. The converters convert a brake pressure pneumatically controlled into the converters by the ABS-valves into a hydraulic brake pressure for assigned vehicle brakes. In the converters, sensors are integrated, which sense physical quantities which occur when a brake pressure defined by the driver is controlled-in. The sensors respond and generate a warning signal when, during the ventilation of the assigned pneumatic/hydraulic converter, a pneumatic piston of the converter is in a stop position. When at least one sensor responds, the brake pressures of the vehicle brakes on both sides of the vehicle axle are limited by the bleeding of the assigned ABS-valves.
The firm brochure xe2x80x9cAntilock Systems (ABS) for Commercial Vehiclesxe2x80x9d, Section 7, WABCO Co., 1992, describes a combined compressed-air/hydraulic braking system, in which case one pneumatic/hydraulic servo cylinder respectively is assigned to the brakes of the rear axle, one ABS valve respectively being connected in front of the servo cylinders. The servo cylinders are pneumatically acted upon via pressure by way of a foot brake valve and the assigned ABS valves. The servo cylinders hydraulically transmit this brake pressure to the rear-axle brakes. The ABS valves are controlled as a function of the signals supplied by rotational wheel speed sensors.
From German patent document DE 44 23 086 A1, a slip-controlled hydraulic braking system is known in the case of which a travel sensor is provided centrally, that is, at the hydraulic master brake cylinder actuated by way of the brake pedal. The travel sensor senses the travel carried out by the brake pedal. When the master brake cylinder volume is almost exhausted although a further brake pressure buildup is desired, the travel sensor will signal that an additional pressure medium volume is required.
From German patent document DE 38 28 9321 A1, a hydraulic braking system is known in the case of which a travel sensor for sensing the piston path is also provided at the master brake cylinder. The travel sensor permits the detection of a disturbance at the braking system.
From German patent document DE 39 21 309 A1, a braking system is known in the case of which the driver defines, by way of the position of a service brake valve, a brake pressure for the vehicle, which is controlled into the ABS valves assigned to the front wheels of the vehicle. The ABS-valves limit the controlled-in pressure such that the vehicle wheels do not lock and transmit the possibly limited pressure to the wheel brake cylinders of the front wheel brakes. In this case, the actually controlled-in pressure is measured by pressure sensors and is processed by an electronic control system.
From the Automotive Handbook/Bosch, 22nd Edition, VDI Publishers, Dusseldorf, 1995, Page 640, it is known to use, in the case of trucks, service brake systems with a pneumatic-hydraulic power transmission, in which case the driver defines a pneumatic brake pressure which is converted to a hydraulic brake pressure for operating the vehicle brakes.
If a leakage occurs in the hydraulic part of the braking system, this leads to a partial or complete brake failure. If two independent hydraulic circuits are provided, for example, at the front axle for the right and the left brake, and if one of these two should fail while the other is operating properly, a yawing moment occurs during a braking operation. This yawing moment, particularly during braking in curves, may result in a swerving of the vehicle.
It is an object of the invention to provide a system which permits a leakage detection in the hydraulic brake system and a stabilizing of the driving condition.
This object is achieved by a braking system for vehicles, particularly commercial vehicles, having a service brake valve which is connected by way of pneumatic lines with ABS-valves of a vehicle axle. Each ABS-valve is connected with an assigned pneumatic/hydraulic converter, which converts a brake pressure pneumatically controlled into the converter by the ABS-valve into a hydraulic brake pressure for an assigned vehicle brake. Sensors are integrated into the assigned converters for sensing physical quantities which occur during the controlling-in of a brake pressure defined by the driver. The sensors respond and generate a warning signal when, during the ventilation of the assigned pneumatic/hydraulic converter, a pneumatic piston of the converter is in a stop position. When at least one sensor responds, the brake pressures of the vehicle brakes on both sides of the vehicle axle are limited by the bleeding of the assigned ABS-valves. The sensors of the pneumatic/hydraulic converters are connected with an electronic brake control unit, which controls the ABS-valves such that, when one of the two sensors responds, the electronic brake control unit limits the brake pressure which is controlled into that vehicle brake which is assigned to the other sensor to a defined maximum pressure. Advantageous developments and further developments are contained in the subclaims.
The basic principle of the invention consists of providing, in the case of a pneumatic-hydraulic braking system with a pneumatic/hydraulic converter which converts a pressure pneumatically controlled-in by the driver into a hydraulic brake pressure, a sensor which senses a physical quantity occurring in the converter when the pneumatic pressure is controlled in, and which generates a warning signal when a defined quantity is sensed.
The physical quantity to be sensed may, for example, be a pressure or a piston position of a piston of the pneumatic/hydraulic converter. From this quantity, a conclusion can be drawn with respect to a leakage in the hydraulic braking system. If such a disturbance is detected, critical driving conditions can be stabilized which occur when the brake is operated and during which, for example, because of a one-sided brake failure, a yawing movement occurs. This can be achieved by means of a partial or complete bleeding of a brake on the opposite vehicle side, which takes place, for example, by way of an ABS-valve assigned to the brake. The vehicle therefore remains on its track in a stable manner even in the event of a one-sided brake failure.
According to a further development of the invention, the pneumatic/hydraulic converter is a servo cylinder with a pneumatic and a hydraulic converter part. In this case, the sensor can be integrated in the pneumatic part of the converter. The sensor may, for example, be an electric switch which will respond only when a pneumatic piston of the converter moves into a stop position or end position when the converter is ventilated, that is, when it is acted upon by brake pressure. Such a position will not be reached when the braking system is intact because then a hydraulic brake pressure builds up which counteracts a further displacement of the piston and limits the xe2x80x9cpiston strokexe2x80x9d. Thus, when such a xe2x80x9ccriticalxe2x80x9d piston position is reached, a clear conclusion can be drawn that there is a leak in the hydraulic part of the braking system.
As an alternative, a pressure sensor may also be provided which senses a hydraulic pressure failure behind the pneumatic/hydraulic converter.
The arrangement according to the invention is preferably provided on both sides of a vehicle axle, particularly the front axle. When one of the two sensors assigned to the front wheels responds and generates a warning signal, this warning signal can be used as an input signal for an electronic brake control system. The control system regulates, for example, the brake pressure at the other front wheel such that a stable driving condition occurs, and increases the brake pressure at the rear wheel such that a residual braking effect is available which is as good as possible.